Different types of electrolytes are being used for recently widely used electrochemical devices, for example, lithium secondary batteries, electrolytic condensers, electric double-layer capacitors and electrochromic display devices, as well as dye-sensitized solar cells of which various studies are being undertaken for future commercialization, and so the importance of electrolytes is increasing day by day.
In particular, lithium secondary batteries are attracting the most attention due to their high energy density and long cycle life. Generally, a lithium secondary battery includes an anode made of carbon material or lithium metal alloy, a cathode made of lithium metal oxide, and an electrolyte obtained by dissolving a lithium salt in an organic solvent.
Initially, lithium metal was used as an anode active material for an anode of a lithium secondary battery. However, because lithium has low reversibility and low safety, currently carbon material is mainly used as an anode active material of a lithium secondary battery. The carbon material has low capacity compared with lithium, but is advantageous in that it has a small change in volume, excellent reversibility, and low price.
As the use of lithium secondary batteries are expanding, the demand for high-capacity lithium secondary batteries are also increasing more and more. Accordingly, there is a demand for high-capacity anode active materials that may substitute the carbon material having low capacity. In order to meet the demand, attempts were made to use metals as an anode active material, for example, Si, Sn, and the like, that have a higher charge/discharge capacity than the carbon material and that allow electrochemical alloying with lithium.
However, this metal-based anode active material has a great change in volume during charging/discharging, which may cause cracks to an active material layer. Secondary batteries using this metal-based anode active material may suddenly deteriorate in capacity and reduce in cycle life over repeated cycles of charging/discharging, and thus, are not suitable for commercial use.
To solve this problem, attempts have been made to use an alloy of Si and other metal or an alloy of Sn and other metal as an anode active material. The use of such an alloy contributes to the improvement of cycle life characteristics and prevention of volume expansion to some extent when compared with the use of metal alone as an anode active material, but the volume expansion generated during alloying with lithium still causes stress, thereby leading to an insufficient commercial use of secondary batteries.